The present invention relates to a method and apparatus for determining the physical integrity of certain geological strata, and, more particularly, to an improved method and apparatus for determining the ability of certain chemical moieties to stabilize geological strata comprising shale by simulating the environment experienced by such stratum and quantatively measuring its physical integrity after exposure to such moieties.
In the exploration for oil and gas, and in the drilling of wells generally, drilling operators use a variety of drilling fluids, or drilling muds, to assist in the drilling process. These muds, which are normally commercial formulations, are selected depending on the geological conditions in the borehole for their ability to stabilize the borehole wall, reduce friction between the borehole wall and the drill pipe, and/or to lift cuttings from the bottom of the borehole. Typical drilling muds have been formulations comprising refined oils and other additives, but oil muds can interfere with well logging procedures and can produce adverse environmental effects if not properly handled. Consequently, drilling operators shifted toward aqueous based drilling muds. However, geological strata, such as certain uncemented or young shales, can lose physical integrity when exposed to aqueous drilling fluids, and this can adversely effect drilling operations. To overcome this problem, research was begun to determine which chemical moities would be most effective at inducing physical integrity in geological shales and to develop aqueous drilling muds containing those moieties.
Some of the test procedures used to evaluate shales are described in "The Effect of Various Polymers and Salts on Borehole and Cutting Stability in Water-Base Shale Drilling Fluids" by G. M. Bol, Koninklijke/Shell E & P Laboratorium, a paper presented at the 1986 IADC/SPE Drilling Conference held in Dallas, Tex., Feb. 10-12, 1986. These tests aid the geologist in determining swelling, erosion and disintegration properties of shales. As can be seen from the schematic representations and the discussion, tests of this kind can require expensive specialized equipment and can be quite time consuming. U.S. Pat. No. 3,646,997 describes a procedure for determining compatibility of a well fluid with a subterranean shale by determining the direction and extent of water migration (due to absorption or desorption within the shale) between the drilling fluid and the shale.
U.S. Pat. No. 4,359,901 describes a method for measuring the chemical swelling effect of a fluid on a shale-containing subterranean earth formation.
A quicker, more easily accomplished bench-scale test is required which will simulate the environment experienced by geological strata during the drilling of a well so that the physical integrity of the strata can be measured quantitatively immediately after exposure to a drilling fluid. This can now be done under controlled physical conditions, including liquid shear, temperature and static pressure.